My $.02 worth!! I have a 30-378 in a sako.
It only shoots well with 150 grain bullets and it doesn't like to shoot with a clean barrel. (seems to need about 10 shots to settle down.) I guess I should just leave the barrel dirty.
ANYHOW- my local gunsmith has bore scoped it and says the barrel looks real good. Can't figure out the fouling.
Well maybe it's just my personal gun but the gunsmith says he's had a quite a few 30-378 owners stop by asking him to "wave his magic accuracy wand" over their gun to make it shoot. I guessthat tells me something.
IN all honesty I don't think a 26" tube works with that big a case. The average 30-378 will not "outrun" a rum with a 26" tube.
I can't help but think the accuracy trouble might be caused by turbulance or incomplete burning of the powder as the bullet exits the muzzle. I've heard some really really astounding accuracy claims from RUM owners so I'd have to point you in that direction.
MAYBE someday I'll get a custom 27-28" tube put on my gun or maybe convert it to a 338-378. OR maybe I'll just stick with my 150 grain xbt's at 3700 fps. I did take a 6x6 last fall at 100 yds diagonally with that bullet. it dropped in it's tracks and had no bloodshot meat. Enough rambling--don't know if I helped--good luck

When I look at the equipment listings in the back of Precision Shooting for 1000 yd BR competitions, I see a lot of chamberings like the 300 Wby Improved or the 308 Baer. I don't remember seeing many 30-378s. I too think the 30-378 needs a really long barrel (30+")to use all that case capacity. (The 300 RUM is very overbore as well). I also see where the long range guys use long heavy bullets with high BCs vs lighter faster bullets with low BCs.

Maybe a good choice would be a #6-7 contour, 28" barreled 300 Win, 300 Wby Imp, or 308 Baer. They will have much better barrel life, will allow you to simply rebarrel your Sendero, and are certainly adequate for long range shooting.

Like I said, all these are being sorted out as we speak. My idea for long range is bullets with BCs in the .5 range, even if they are slow. I can laser range the distance and know exactly, so I know how much to hold over or crank into my elevation knob. However, the wind is always a guess, so I want a bullet that doesn't drift as much in the wind in case I misjudge. This is nothing I came up with, it's what the long range target shooters do.

As a side note, my intial deer load for my 30-06 is a 180 grain Sierra Gameking (BC .5) at a chronographed MV of 2880 fps. I was in a rush to get ready for this season, so I haven't fully sorted out the load. Even so, it is averaging .6 moa at 100 yds. I haven't had time to test it at longer ranges yet, but my expereince with long bullets is that they moa better at 200 and 300 yds than they do at 100. When you look at the ballistics data for a .5 BC bullet with an MV of 2880, it is amazing how flat and wind resistant it is--much more than I thought at fist glance.

#1 There is too much free bore for one thing.
By the time you seat the bullet out to TRY to touch the lands, you will never be able to use the magazine because of the extended length of the case. You may never be able to touch the lands anyway.
#2 The barrels need to be longer to shoot the heavy bullets with a high BC.
They advertise a 28" barrel but they actually have a 26" barrel with a muzzle brake. As we all know, the muzzle brake does not add barrel length. As a matter of fact the bullet does not (or better not) touch any part of the brake. There should be .016" free space completly around that bullet when it passes through the brake.
Therefore, it is not part of the overall length of useable barrel.

#3. Check out the bolt locking lugs on a factory Weatherby action sometime. See if they are all locking evenly. I'll bet they are not. A gunsmith can correct most of these problems to make the gun shoot.

There are ways to accurize the Weatherby rifles chambered in the 30/378 to make it shoot but, right from the factory they are known to be VERY inaccurate.

Cut the barrel back a few threads rechamber and get rid of the excessive free bore is a start. Put a new custom LONG barrel on it is another way.

All my custom 30/378s had at least 36" barrels on them.

Howard Wolfe, the noted gunsmith and his whole shooting family at Williamsport shoots nothing but the 30/378 caliber Heavy guns with long barrels and 220 and 240 gr bullets.
He has shot this caliber in the 1000 yard maches since he designed it many years ago.

Why have a longrange rifle with a chambering of a big overbore case and shoot only light bullets in it? Longrange shooting or I should say Longrange hunting should be done with the most accurate and highest BC bullet available in the caliber you are shooting to retain the velocity and energy at extended range.